Although methane is abundant, its relative inertness has limited its utility in conversion processes for producing higher-value hydrocarbons. For example, oxidative coupling methods generally involve highly exothermic and potentially hazardous methane combustion reactions, and frequently require expensive oxygen generation facilities and produce large quantities of environmentally sensitive carbon oxides. Non-oxidative methane conversion is equilibrium-limited, and temperatures ≧about 800° C. are needed for methane conversions greater than a few percent.
One way to avoid this difficulty involves converting methane to a mixture comprising carbon monoxide and molecular hydrogen (the mixture being conventionally referred to as “syngas”), converting the syngas to a mixture of oxygenates, and then converting the oxygenates to olefins. See, e.g., U.S. Patent Application Publication Nos. 2005/0107481 A1, 2008/0033218 A1, and 2007/0259972 A1, which disclose aspects of converting syngas to a mixture comprising methanol and ethanol, and then converting the mixture to a product mixture comprising ethylene and propylene. The mixture's methanol, in contrast, produces (i) ethylene and propylene, in approximately equal amounts, and (ii) a significant amount of by-products. Besides the desired methanol and ethanol, the process also yields relatively low-value by-products such as molecular hydrogen, water, carboxylic acids, ethers, carbon oxides, ammonia and other nitrogenated compounds, arsines, phosphines, and chlorides. Relatively low-value hydrocarbon by-products are also produced, such as acetylene, methyl acetylene, propadiene, butadiene, butylene, and the like. It is desired to decrease by-product yield, particularly hydrocarbon by-product yield, and increase oxygenate yield, particularly C2+ alcohol yield, and more particularly C3+ alcohol yield.
Conventional processes for increasing C2+ alcohol selectivity include those disclosed in P.C.T. Patent Application Publication No. WO 2012/078276 A1. The reference discloses a heterogeneous catalytic process for producing ethanol and propanol from syngas. Although the reference's example report an ethanol selectivity of up to 22.4% and appreciable n-propanol selectivity (up to 7.6%), the process also produces a significant amount of methane (selectivity of up to about 13.8%). Homogeneous processes, such as those disclosed in U.S. Pat. Nos. 4,265,828; 4,605,677; and 8,912,240 have an increased selectivity for oxygenated products in comparison with the heterogeneous processes. Although methane yield is decreased, representative homogeneous processes, such as those disclosed in U.S. Pat. No. 4,622,343, exhibit appreciable methanol yield in comparison to their yield of more desirable ethanol. The amount of ethanol can be increased by homologation of recycled methanol, as disclosed in G. Srinavis, J. Martin, S. C. Gebhard, and M. V. Mundschau, Prepr. Pap.-Am. Chem. Soc. Div. Energy Fuels 2013, 58 (2). Similarly, U.S. Pat. No. 4,935,547 discloses recycling methanol or higher alcohols for homologation to produce higher boiling alcohols. It is also conventional to add gaseous alcohol to the gaseous syngas feed when producing ethyelene glycol (U.S. Pat. No. 4,265,828) or C1-C4 alcohol (U.S. Pat. No. 4,622,343).
Flexible processes are now desired, which can produce C2+ oxygenates over a wide range of relative amounts, but with a lesser methane yield than conventional heterogeneous alcohol synthesis processes and a lesser methanol yield than conventional homogeneous alcohol synthesis processes. More particularly, processes are desired which have an increased yield of C3+ alcohol over conventional processes, and which also make useful by-products such as C2+ glycol.